Heterogeneous catalysts have been shown to be useful in enhancing the rate and/or efficiency of gas phase reactions in a number of applications. These applications include emerging technologies, such as catalytic reactors or fuel reformers that are used to produce hydrogen gas, H.sub.2, from hydrocarbon fuels, such as gasoline, natural gas, and alcohols, as well as relatively mature technologies, such as the catalytic convertors used to reduce the emission of pollutants from automobile and truck engines. The performance of heterogeneous catalysts may be severely degraded by exposure to catalyst poisons, such as the sulfur and phosphorous compounds that are found in varying amounts in automotive fuels, such as gasoline. As gasoline is expected to be used, at least initially, in automotive applications of fuel cells, the possible poisoning of both fuel cell catalytic reactors, automotive catalytic convertors, and other catalytic combusters by fuel contaminants is a major concern regarding the effectiveness of these devices.
Fuel cells are electrochemical devices that convert the chemical energy of a fuel directly into electrical and thermal energy, and have been used for a number of years in aerospace applications, such as the space shuttle, where hydrogen and oxygen gas are combined to produce electric power. In a typical fuel cell, a gaseous fuel, e.g., hydrogen, H.sub.2, is fed continuously to an anode or negative electrode compartment, and an oxidant, e.g., oxygen or an oxygen containing gas, which is typically air, is fed continuously to a cathode or positive electrode compartment. The hydrogen and oxygen are combined at the electrodes, producing water and an electric current. In addition to water, fuel cells that utilize catalytic reactors to produce hydrogen gas from hydrocarbon fuels also release carbon dioxide, and may also release very small amounts of carbon monoxide.
Theoretically, a fuel cell is capable of producing electrical energy for as long as the fuel and oxidant are supplied to the electrodes. However, pure hydrogen is difficult to store, particularly in a vehicle, and its use may not be practical in many applications. In those cases, a catalytic fuel reformer may be used to produce hydrogen gas from a hydrocarbon fuel, and, thus, the life and performance of the fuel cell is limited by the performance and efficiency of the catalytic reactor. As discussed above, if one or more catalyst poisons are present in the fuel used to produce hydrogen in the catalytic fuel reformer, the performance of the reformer will be degraded, thereby reducing the performance of the fuel cell.
In addition, fuel cells are sensitive to carbon monoxide, and, thus, the amount of carbon monoxide is typically minimized in the fuel gas by removal by the catalytic reactor to achieve optimum efficiency of the fuel cell. However, where the catalyst is contaminated or poisoned, carbon monoxide will remain in the fuel gas after passing through the catalytic reactor. Therefore, for the fuel cell to function efficiently, the catalyst should be substantially free of poisons that prevent the removal of carbon monoxide from the fuel gas.
Similarly, in virtually all modern gasoline engines used in vehicles, such as automobiles and light trucks, the exhaust gases produced during combustion of fuel are conveyed by an exhaust pipe to a catalytic converter where pollutants, such as carbon monoxide (CO), hydrocarbons (HC), and oxides of nitrogen (NO.sub.x), are substantially converted to non-polluting species, and, thus, are removed from the exhaust gas. In addition, it is expected that catalytic convertors will soon be developed for use with diesel engines. Most modern engines employ three way catalytic converters ("TWC"), which simultaneously oxidize CO and HC to CO.sub.2 and H.sub.2 O, and reduce NO and NO.sub.2 to N.sub.2. The amount of CO, HC, NO.sub.x and other pollutants produced will vary with the design and operating conditions of the engine and the fuel and air used. In particular, as with fuel cell catalytic reactors, the presence of catalyst poisons in the fuel will result in a degradation of the performance of the catalytic convertor, and, thus, an increase in the amount of pollutant released into the air.
In general terms, a catalytic convertor used with an internal or external combustion engine may be considered to be a sophisticated catalytic combuster, which is typically used to enhance the oxidation of a fuel to produce heat. The heterogeneous catalyst in a catalytic combuster provides a surface on which a fuel and an oxidizer react. In a typical catalytic combuster, a vaporized fuel and air are passed over the surface of the catalyst. By providing a catalytic site for the reaction of the fuel and oxidizer, the catalyst lowers the activation energy of the reaction, allowing the reaction to occur at a lower temperature with greater efficiency. However, the presence of catalyst poisons that may be adsorbed onto the catalyst surface in any of the fuel, oxidizer, or reaction products will degrade the performance and the efficiency of the catalytic combuster by occupying active sites on the catalyst surface. This reduces the number of sites available to the fuel and oxidizer, decreasing the reaction rate.
In general terms, the heterogeneous catalysts, used in fuel cell catalytic fuel reformers or reactors, vehicle catalytic convertors, and catalytic combusters, provide a catalytic surface that enhances the reaction rate and efficiency of various gas phase reactions. Although a number of different heterogeneous catalysts are known, the heterogeneous catalysts used in catalytic reactors and catalytic convertors usually utilize a noble metal catalyst. The structure of the catalyst support may vary, depending on the application, e.g., ceramic beads that are coated with the catalytic material may be used. However, where a large throughput of gas is required, the noble metal catalyst is preferably held in a honeycomb monolithic structure, which has excellent strength and crack-resistance under physical and thermal shock.
The honeycomb construction and the geometries chosen provide a relatively low pressure drop and a large total surface area that enhances the mass transfer controlled reactions that produce fuel for the fuel cell or remove pollutants from the exhaust of an engine. The honeycomb is often set in a steel container, and protected from vibration by a resilient matting where needed. Although a single catalyst may be use, a typical modern three way catalytic convertor comprises an outer steel shell that contains at least two honeycomb catalyst "bricks", i.e., honeycomb monolithic structures holding the noble metal catalyst, as described above, where one of the bricks is mounted at the upstream, inlet end of the catalytic convertor, and the second is mounted at the downstream, outlet end of the catalytic convertor.
An adherent washcoat, frequently made of stabilized gamma alumina or corderite into which the catalytic components are incorporated, is deposited on the walls of the honeycomb. Modern three way catalytic converters for simultaneously converting all three pollutants typically utilize the precious or noble metals platinum (Pt) and rhodium (Rh), where the Rh is most responsible for the reduction of NO.sub.x, while also contributing to CO oxidation, which is primarily performed by Pt. Recently palladium, Pd, which is less expensive, has been substituted for or used in combination with Pt and Rh. The active catalyst generally comprises about 0.1 to 0.15% of these metals. For other applications, where reduction of NO.sub.x is not required, so that only the oxidation of CO or HC are required, rhodium is typically not present in the catalyst. Instead the catalyst is platinum, palladium, or a combination of platinum and palladium.
Because the exhaust gases of the combustion process in most modern automotive gasoline engines tend to oscillate from slightly rich to slightly lean, an oxygen storage medium is added to the washcoat of vehicular catalytic convertors to adsorb oxygen onto the surface of the washcoat during any lean portion of the cycle, and release the oxygen for reaction with excess CO and HC during any rich portion of the cycle. Cerium Oxide (CeO.sub.2) is frequently used for this purpose due to its desirable reduction-oxidation response.
The conversion efficiency of a gas phase reaction heterogeneous catalyst is measured by the ratio of the rate of mass conversion or removal of a particular constituent of interest to the mass flow rate of that constituent into the catalytic. The conversion efficiency of a catalyst is a function of many parameters including aging, temperature, stoichiometry, the presence of any catalyst poisons, such as lead, sulfur, carbon and phosphorous, the type of catalyst, and the amount of time the gases reside in or on the catalyst.
As discussed above, catalyst poisons, such as sulfur and phosphorous, degrade the performance of catalysts. The performance of catalytic convertors, catalytic fuel reformers or reactors, catalytic combustors of various types, and heterogeneous catalysts in general are affected by such poisons. Poisons, even in small concentrations, strongly bond to catalytic sites on the surface of the catalyst, and block the completion of the chemical processes that the catalyst is intended to promote. The poisoning of vehicular catalytic convertors by sulfur in gasoline has been a problem, and is expected to also be a severe problem in fuel cell catalytic reactors that are proposed for automotive applications, where the required hydrogen gas will, in all likelihood, initially be produce from gasoline.
The issue of catalyst poisoning is not new. For example, the effectiveness of automotive catalytic convertors is severely degraded by the presence of lead in gasoline. Therefore, the introduction of catalytic convertors on production automobiles in the mid-1970's required the elimination of tetra-ethyl lead as an octane enhancer in fuels. Although the elimination of the lead based octane enhancer required research into alternative octane enhancers, it did not require any major changes in the manner in which the fuel itself is refined, and, thus, the cost of eliminating tetra-ethyl lead from gasoline was not prohibitive. However, the elimination of sulfur, a naturally occurring element in crude oil, from fuel may be far more expensive.
Now that lead has been essentially eliminated from motor vehicle fuel in the United States, sulfur is the key component in gasoline responsible for the poisoning of catalysts. Sulfur, typically adsorbed in the form of oxides of sulfur, attaches or binds to catalytically active areas on the surface of the catalyst, such as those used in catalytic combusters, catalytic convertors, and catalytic fuel reformers or reactors. The adsorption of at least one of sulfur and sulfur compounds prevents the resulting poisoned areas from participating in the gas phase reaction, such as the oxidation of HC and CO, and the reduction of NO.sub.x in an automotive catalytic convertor, and thereby reduces the efficiency of the catalyst. As a result, the emission of pollutants is increased where the catalyst is used in an internal combustion engine catalytic convertor. Similarly, it is expected that the presence of sulfur in gasoline will degrade the performance of catalytic reactors used to produce hydrogen from gasoline to be used as fuel in a fuel cell.
The sulfur content of gasoline presently varies from state to state and from refinery to refinery. Where California has a limit on gasoline sulfur content of approximately 30 parts per million by weight ("ppm"), other states have much higher limits on sulfur, and, as a result, sulfur levels in fuel can exceed 900 ppm. Therefore, there has been a push within the Environmental Protection Agency ("EPA") to set a national standard for gasoline sulfur content. However, even at the proposed level of 80 ppm, a degradation of the performance and efficiency of catalytic convertors and catalytic reactors using a fuel containing that level of sulfur is expected.
Alternative methods for reducing sulfur poisoning of heterogeneous catalysts are available. For example, the catalyst may be heated to a temperature significantly higher than the normal operating temperature to decompose and/or drive off certain poisons, and thereby recover the poisoned catalyst. However, the high temperature required can significantly reduce the life expectancy of a catalytic device, and is frequently not possible during normal operation.
Attempts to remove the sulfur compounds that poison catalysts from the gas or exhaust stream before poisoning of the catalyst occurs by direct filtering or by oxidation of SO.sub.2 to SO.sub.3, either catalytically in the presence of oxygen, i.e., lean conditions, or in a plasma discharge, have been largely unsuccessful. While each of these methods has been explored for automotive applications, they often fail to remove any significant amount of sulfur or oxides of sulfur, and require significant amount of power. Moreover, these methods may not be feasible with fuel cell catalytic fuel reformers at all.
Therefore, a need exists for a simple, inexpensive means of maintaining the efficiency of gas phase heterogeneous catalysts, such as those used in automotive catalytic convertors, fuel cell catalytic reactors, and catalytic combusters. The present invention provides such a means.